[31.06] Constraints on the Mass of Planets with Water of Nebular Origin

M. Ikoma, H. Genda (Tokyo Tech)

Detection of many extrasolar planets has stimulated us to
make a systematic study of planet formation. Theoretical
studies on the accretion and dynamical evolution of planets
have constrained the masses and periods of planets in
extrasolar systems. From an astrobiological point of view,
special attention has been paid to probabilities of the
existence of planets in the habitable zone where a planet
can keep liquid water above its surface. Few studies have,
however, discussed how likely a planet acquires a sufficient
amount of water. Although there are several sources of water
on terrestrial planets, we focus on an idea that water is
produced on a planet by oxidation of a hydrogen-rich
atmosphere, the nebular gas being attracted gravitationally
by the planet. The process of water production could work if
a planet captures a sufficient amount of hydrogen, has a
molten surface (i.e., magma oceans), and contains some
oxide. Our extensive investigation of properties of the
hydrogen-rich atmosphere shows the former two conditions are
fulfilled on an Earth-size planet for a wide range of
parameters. Moreover, some oxide such as FeO is common
material in planets, as long as the C/O ratio of extrasolar
systems is less than unity. Therefore, sufficient water on
an Earth-sized rocky planet is a natural consequence of
planet formation. The range of masses of those
potentially-habitable planets is also constrained.

This research was supported by the 21st Century COE Program
``How to build habitable planets", Tokyo Institute of
Technology, sponsored by the Ministry of Education, Culture,
Sports, Technology and Science (MEXT), Japan.